The eukaryotic cell is a very complex biological entity which undergoes dramatic structural and functional changes during the processes of cellular activation, differentiation and transformation. Human tumor cells are distinguished from their normal counter parts by the expression of proteins associated with the various respective genetic or epigenetic changes. High resolution two-dimensional (2D-) polyacrylamide gel electrophoresis which is capable of the simultaneous resolution of thousands of individual proteins has proven to be an important technique in the analysis of cellular protein expression. The enhanced resolution obtained by coupling 2D-gel resolution with different immunological and biochemical techniques has made it feasible to identify novel polypeptide gene products as well as post-translational modifications. Furthermore, with the use of computerized gel scanning and image processing it is possible to examine the patterns of gene expression of a given cell type, assign number to individual polypeptides, compare, quantitate and store the wealth of information contained in the gels. The LCRC 2D-gel facility is equipped with state-of-the-art technology and expertise for (1) the resolution of complex protein mixtures using ISO- DALT methods and subsequent immunological or biochemical analysis; (2) the capacity to perform computerized data analysis with the aid of ELSIE-5 program. During the past year, five peer reviewed funded projects have used the 2D-gel services and made substantial progress. Many additional projects plan to use the facility in the coming year. Currently, our approaches are directed to address topics such as I. Differential protein expression in (a) radioresistant vs. radiosensitive human tumors cells, (b) radiation-transformed human epithelial cells, (c) estrogen-responsive and estrogen-independent breast cancer cells; and II. Post-translational modifications (a) in response to DNA damage/repair with particular emphasis on ADP-ribosylation, (b) as mediators of cellular invasion in transformed cells in the form of tyrosine phosphorylation.